CID:1214196 Dereference null return value, if we hit this condition, something very bad has happened, we should never have the private not set if we get to the hangup hook, log error instead of crashing

CID:1214163 Unchecked return value, check return value of switch_ivr_play_file and bail out if it fails

Putting `net-snmp-config --cflags` into CFLAGS causes major pollution;
it overrides optimization and debugging levels, warnings, and more.
While normally we do want to automatically locate library headers,
there has to be a better way to do this.  libsnmp is normally in the
usual place and doesn't need special handling.  Perhaps people with
libsnmp in a weird place should just need to add the -I flag to their
CFLAGS before build.

Calling out to net-snmp-config --agent-libs causes transitive
dependencies to get pulled in, but we don't need those -- a sensible
dynamic linker pulls those in automatically.  Trying to track the
transitive dependencies manually would be a losing battle.

People were recently hitting this on Debian sid/jessie, where libpci
is in the transitive dependency list but isn't otherwise one of our
build dependencies.

FS-6517 skypopen: mod_skypopen not using configured context with mod_sms chatplan with PATCH, thanks Russell Treleaven

This allows someone without pkg-config in their PATH to set where
pkg-config is located.

We depend on pkg-config, but we weren't explicitly testing for it, so
the absence of pkg-config would cause the sqlite test to fail, which
was confusing.

This makes portable the fix from commit
28ee7736.